Certain abrasive tools for industrial applications usually have an abrasive portion of grains embedded in a bond. This abrasive portion is normally affixed to a rigid core. The core can be adapted for manual or power driven motion in contact with a work piece to grind, cut, polish or otherwise abrade the work piece to a desired shape.
Among other things, the abrasive grains should be harder than the material being ground to penetrate the surface and to remove chips from the work piece. Very hard, so-called "superabrasive" substances, such as diamond and cubic boron nitride ("CBN"), are especially useful for cutting hard or difficult to cut materials. For example, diamond can grind tungsten carbide, natural stone, granite, concrete and ceramics. Diamond is not well suited for grinding iron or steel, however. Importantly, CBN can cut ferrous materials.
Because superabrasives are relatively expensive, it is economically advantageous to reduce the amount of superabrasive on a grinding tool. In one type of abrasive tool (a "Single Layer" abrasive tool) a very small amount of abrasive is deposited in a substantially one grain thickness layer on the operative surface of the core and the abrasive grain is bonded to the core by a metal bond. This bond can be achieved by such methods as electroplating and brazing. Of these two methods, brazing is preferred because electroplating generally requires maintaining a large inventory of expensive superabrasive grains in an electroplating bath.
Sometimes the metal bond can be the service life determining factor for a Single Layer abrasive tool. Composition of the bond affects its bonding strength. Unless the bond is strong, repetitive impact against the work will tear superabrasive grains from the core prematurely, i.e., while the superabrasive grains remain sharp and capable of further cutting. The bond also is normally softer than the work piece. Direct contact with the work piece or with swarf can erode the bond which also permits early release of sharp particles.
Recent technological developments have sought to improve the strength of brazed bonds. For example, U.S. Pat. No. 4,968,326 discloses a method of making a Single Layer diamond abrading tool with good bond strength that can be varied to desired degree. The method employs a brazing material containing a carbide forming element, preferably molybdenum or iron. The patented method also has the stated advantage that the carbide and braze layers tend to climb up the side of the diamond particles. This surface "wetting" phenomenon increases the interface between abrasive particle and bond on which the bond may act, and thus strengthens the bonding power of the braze. In U.S. patent application Ser. No. 08/693,763 filed Aug. 7, 1996, it has been proposed to include in a bronze-based braze, particles of active components, such as titanium, zirconium, titanium carbide, or mixtures of them. These active components can react with the abrasive particle at the surface to form a stronger chemical bond.
Adding active metal such a titanium to the bond composition has a disadvantage. The additive can react with other elements in the composition during brazing to form intermetallic compounds. These intermetallics are weaker than the braze and dilute the remaining braze that is present. Thus the intermetallic compounds detract from the mechanical properties of the braze. Additionally, the intermetallics can adhere the braze very strongly to the metal of the core. This adhesion makes chemical or electrochemical stripping of the braze from worn out tools more difficult. Stripping is an important process in the recovery of recycled tool cores. The ability to recover used cores increasingly affects tool production cost, particularly with respect to large tools for the construction industry, such as large diameter grinding wheels for ferrite.
Consequently, it is desirable to incorporate active metal in the braze composition to strengthen the bond; however, it is also advantageous to minimize active metal in the braze composition to reduce the formation of intermetallics. It now has been discovered that strong bonds for Single Layer abrasive tools can be made with greatly reduced amounts of active component, e.g., 0.5 to 3.0 wt % of the braze composition. The discovery involves use of superabrasive grains coated with a mechanically bound layer of a first active component together with a braze composition containing a second active component. The total amount of active component present in the resulting brazed composition is much less than necessary for conventional bonds made by only incorporating an active component in the braze composition. While creating a strong bond to the superabrasive, the resulting brazed composition leaves little active component available for intermetallic formation and thereby strengthens the bond and facilitates stripping the braze from worn tools.